The invention relates to a photographic apparatus, for use in medical radiodiagnostics, which utilizes a 3-layer system lying between two electrodes which are energized with a d-c voltage. The three layers consist of a material in which charge carriers can be stored sandwiched between two photoconductors.
The "Journal of Applied Photographic Engineering", Vol. 4, No. 4, 1978, pages 178-182, discloses photographic apparatus in which a double layer, consisting of a selenium layer in an electrically insulating layer is used. The two layers are between the bottom electrodes and a transparent covering electrode. In every case the selenium layer must simultaneously fulfill two functions. For absorption of the roentgen radiation, i.e., for taking X-ray pictures, the selenium should be as thick as possible. However, for loss-free charge carrier transport in the point-by-point scanning with a light beam during read-out of the signal it should be as thin as possible. Consequently, the attainable signal strength and the read-out time necessary for the generation of the signal are inferior as a result of this selenium layer.
When this photographic principle is used in medical radiodiagnostics, the X-ray dose is limited to protect the patient. When photographing with such a limited a dose, however, the selenium layer discharges by only about 10%. Accordingly another disadvantage results from the proposed read-out principle. During read-out complete discharge with light takes place, so that the preceding X-ray exposure appears only as a small differential signal against the strong signal background present due to the charge.
U.S. Pat. No. 3,069,551 discloses a system consisting of several layers in which a photoconductor layer sensitive to light as well as one sensitive to X-rays abut a mosaic layer with charge storage capacity. Such an arrangement has the disadvantage, however, that such a mosaic layer which is sufficiently homogeneous for radiodiagnostics is not available.
To obtain a strong video signal with an improved noise ratio, for the system disclosed in U.S. Pat. No. 3,069,551, a homogeneous electrically insulating layer, in accordance with German patent application 3151155.4, can be placed between the X-ray photoconducting layer and the light photoconducting layer in place of the mosaic. In this arrangement the charge carriers are generated by the radiation to be imaged and they must be retained for storage at the photoconductor-insulator interface. However, production of interfaces with equal properties is technologically difficult and therefore, this approach is not practical. Upon read-out, the charge carriers from the light photoconductor layer cannot neutralize the charge carriers stored in the form of images in mere traps, because the insulating layer lies therebetween and by definition does not transmit any charge carriers. To prevent the formation of ghost images in subsequent takes, an effective erasing process must be carried out after each take.